Protective screens of the general type outlined above are well known and many examples appear in the patent literature. Among these there may be mentioned the U.S. Pat. Nos. to Willson 3,051,935; Callaghan 3,706,090; Purdin 2,547,283; Wilson 4,232,310; Minton 3,863,242; Siegerdt 1,630,808; Winter 1,099,480; Dunn et al. 3,696,373; Williams 4,293,778; Kohler 1,172,771; Estes 2,205,945; Larned 181,078; Miller 3,725,891; Pastore 1,252,834; Di Giovanni 1,031,535.
Many of these devices are adequate, particularly the more recent ones. Nevertheless, all suffer from one disadvantage or another, and many suffer from plural disadvantages, including: complex circuitry; complex and unduly expensive construction; mains power supply; difficult installation; difficult replacement of a damaged screen; inability to sense all types of ingress; inconvenience.